[GHC] #15533: Access the number of bits in the target machine's Int type at compile time

17 Aug 2018

      #15533: Access the number of bits in the target machine's Int type at compile time
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           Reporter:  ChaiTRex       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I'm not sure of the current progress on GHC cross-compiling user programs,
 but I think a small addition might need to be made somewhere in the
 modules included with `ghc`.

 If I'm reading the documentation correctly,
 [https://hackage.haskell.org/package/base-4.11.1.0/docs/GHC-
 ByteOrder.html#v:targetByteOrder GHC.ByteOrder.targetByteOrder] will let
 us know the endianness of the target machine even if the compiling machine
 has a different endianness. I assume that this allows Template Haskell to,
 at compile time, generate proper code specifically for the target based on
 that fact.

 I can't find any way of finding out the bit size of an `Int` (specifically
 the exact value of `(finiteBitSize (undefined :: Int)`) on the target
 platform, and it would be nice if that were added in a way that's
 convenient to access in general and by Template Haskell at compile time,
 similar to `GHC.ByteOrder.targetByteOrder`.

 ----

 I've looked into it a little bit and found that
 [https://github.com/ghc/ghc/blob/master/compiler/utils/Platform.hs#L33 the
 GHC API will tell you the word size of the target in bytes with
 platformWordSize] (also on
 [https://hackage.haskell.org/package/ghc-8.4.3/docs/Platform.html#v:platformW...
 Hackage]):

 {{{#!hs lineno=25 id=b marks=31-33
 -- | Contains enough information for the native code generator to emit
 --      code for this platform.
 data Platform
         = Platform {
               platformArch                     :: Arch,
               platformOS                       :: OS,
               -- Word size in bytes (i.e. normally 4 or 8,
               -- for 32bit and 64bit platforms respectively)
               platformWordSize                 :: {-# UNPACK #-} !Int,
               platformUnregisterised           :: Bool,
               platformHasGnuNonexecStack       :: Bool,
               platformHasIdentDirective        :: Bool,
               platformHasSubsectionsViaSymbols :: Bool,
               platformIsCrossCompiling         :: Bool
           }
         deriving (Read, Show, Eq)
 }}}

 The problem is that this doesn't account for possible tag bits reducing
 the size ''in bits'' of `Int` to something less than the given
 `platformWordSize`.

 Recently, [https://ghc.haskell.org/trac/ghc/ticket/15486 support for
 WORD_SIZE_IN_BITS < 32 was dropped], so tag bits aren't a problem on
 32-bit platforms anymore, but they might be on 64-bit platforms.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15533
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